1. Field of the Invention
The present invention relates to a magnetic head that has a spin valve sensor with an improved seed layer for a free layer and more particularly to a seed layer that modifies the micro structure of the free layer for improving a magnetoresistive effect of the sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree. to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is typically parallel to the ABS, is the position of the magnetic moment of the free layer with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is at a minimum and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering in response to field signals from a rotating disk changes the resistance of the spin valve sensor as a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layers and R is the resistance of the sensor when the moments are parallel.
Over the years a significant amount of research has been conducted to improve the GMR or magnetoresistive coefficient dr/R of spin valve sensors. These efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes. A factor in improving the magnetoresistive coefficient dr/R is the texture of the various layers of the spin valve sensor. Researchers strive to optimize the texture of the layers, which is sometimes more texture and sometimes less texture, for improving the layers performance. The performances of the spacer layer and the free and pinned layers on each side of the spacer layer are the most important for improving the magnetoresistive coefficient dr/R. This is where the scattering of conduction electrons takes place and an increase in phased scattering equates to an increase in the magnetoresistive coefficient dr/R.
The spin valve sensor is made by sputter depositing the layers consecutively on the first read gap layer. The first read gap layer, which is typically aluminum oxide (Al.sub.2 O.sub.3), affects the texture of the first layer of the spin valve sensor deposited thereon. Sometimes this affect is not favorable or needs improvement. For this reason a seed layer is often located between the first read gap layer and the next layer of the spin valve sensor for favorably modifying the texture of the next layer.
The type of seed layer employed on the first read gap layer depends somewhat on whether the spin valve sensor is a top spin valve sensor or a bottom spin valve sensor. In a top spin valve sensor the pinning layer is located in a top portion of the spin valve sensor close to the second read gap layer and the free layer is located in a bottom portion of the spin valve sensor close to the first read gap layer. In a bottom spin valve sensor the pinning layer is located in a bottom portion of the spin valve sensor close to the first read gap layer and the free layer is located in a top portion of the sensor close to the second read gap layer. While the free layer can be deposited directly on the first read gap layer in a top spin valve sensor it has been found that a seed layer between the first read gap layer and the free layer can be employed for favorably modifying the texture of the free layer for improving the magnetoresistive coefficient dr/R. The search continues for seed layers which will promote the best magnetoresistive coefficient dr/R so as to increase storage capacity.